Increased sophistication in aircraft has resulted in so-called "fly-by-wire" aircraft wherein the controls operated by the pilot and control surfaces responsive to the controls are not directly mechanically coupled. Rather, the same may be coupled by electrical or hydraulic systems. That in turn requires that electrical or hydraulic energy be available at all times or else the link between the controls and the control surfaces is lost.
Consequently, such aircraft require advanced airborne secondary power systems. Particularly when used in military applications, such secondary power systems must be capable of operation at remote sites with a minimum use of special ground support equipment. In addition, the same must be capable of virtually instant start up from an inactive position, whether at sea level or at extremely high altitudes in excess of 50,000 feet.
Frequently the power systems include power units known as auxiliary power units or emergency power units or combinations of both. These units generally provide useful work as an output from a turbine engine which in turn is normally driven by gases of combustion.
While this presents no difficulties in most instances, at high altitudes, where the air is least dense meaning that a given volume has a minimal quantity of oxygen, starting of such power units in such a way that energy therefrom will be available to operate control surfaces in response to control commands within an elapsed time of two or three seconds, difficulties are encountered.
One prior art proposal for avoiding the difficulty includes the use of hydrazine which is stored on board the aircraft. The hydrazine is applied to a catalytic chamber wherein it decomposes and generates gas under pressure sufficient to drive a turbine wheel and accelerate the same to the point where its operation can become self sustaining at altitude. While this works well, storage of unstable material such as hydrazine aboard aircraft is not desired and the same requires special handling and may not be readily available for use at remote sites.
The present invention is directed to overcoming the above problems.